JP2012042740A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of suppressing slippage between a drive roller and an intermediate transfer belt upon changing a speed and capable of forming a favorable image.SOLUTION: The image forming apparatus carries out steps of primarily transferring the whole toner image onto an intermediate transfer belt, changing a speed of the intermediate transfer belt in accordance with a type of a recording material, then secondarily transferring the image onto the recording material after the toner image again passes the primary transfer position while held on the intermediate transfer belt. Upon changing the speed of the intermediate transfer belt, the image forming apparatus controls one of or both of a voltage applied to the primary transfer member and a surface potential of a photoreceptor so as to control the potential difference between the primary transfer member and the photoreceptor surface to be smaller than the potential difference when the toner image on the intermediate transfer belt passes at the primary transfer position, and then changes the speed of the intermediate transfer belt.

Description

  The present invention relates to an electrophotographic image forming apparatus such as a printer, a FAX, and a copying machine.

  FIG. 7 shows a full-color image formation, which is well known to those skilled in the art and includes a charging unit 2, an exposure unit 3 and a developing unit 50 around an electrophotographic photosensitive member 1, and forms a toner image on the photosensitive member 1 by an electrophotographic process. An example of an apparatus is shown. In the full-color image forming apparatus as in this example, the toner image on the photosensitive member 1 is temporarily transferred onto the intermediate transfer belt 16 as a transfer target, and then primary transferred, and thereafter, the toner image on the intermediate transfer belt 16 is transferred to 4 on the intermediate transfer belt 16. The color toner image is secondarily transferred onto the final transfer material P at once.

  The intermediate transfer belt 16 includes a plurality of rollers 16a, 16b, and 16c inside the belt, and the belt 16 is stretched by them. One of the rollers is a driving roller 16a. The drive roller 16a is rotationally driven to rotate the belt 16. A frictional force acts between the driving roller 16a and the belt 16, and the belt 16 is conveyed following the driving roller 16a by this force. In addition, a primary transfer member 20 is provided inside the belt at a primary transfer position B where the photoreceptor 1 and the intermediate transfer belt 16 are in contact with each other. The primary transfer member 20 is pressed against the photoreceptor 1 with the intermediate transfer belt 16 interposed therebetween. A voltage is applied to the primary transfer member 20. The toner image on the photoreceptor is primarily transferred to the intermediate transfer belt 16 due to a potential difference between the surface of the photoreceptor and the intermediate transfer belt.

  In order to cope with the diversification of users in recent years, the speed of the secondary transfer process and the fixing process is changed according to the paper type. Conventionally, for example, when a thick paper or an OHT sheet is used as the final transfer material P, it is known that the process speed of the secondary transfer process and the fixing process is reduced by about half compared to the case of using plain paper. This is because when the toner is secondarily transferred to a transfer material P such as thick paper, the electric field is smaller than that of plain paper and transfer failure occurs. Further, when fixing, since the heat transfer is weaker than that of plain paper, fixing failure occurs. Therefore, this problem is addressed by reducing the speed and increasing the time for passing through the nip.

  As one of the methods for changing the speed at the time of fixing, the process is performed at a predetermined process speed until the primary transfer, and after transferring all the toner images to the intermediate transfer belt, the process speed is switched, the secondary transfer process and the fixing process. The method of doing is known. When changing the process speed, the speeds of the photosensitive member, the intermediate transfer belt, and the fixing device are changed. At this time, in the configuration in which the distance between the primary transfer and the secondary transfer is shorter than the image size, the end of the toner image passes through the secondary transfer portion when the primary transfer of the entire toner image is completed. End up. Therefore, after the primary transfer is completely completed, the speed of the intermediate transfer belt and the fixing device is switched, and the intermediate transfer belt is idled for one rotation while the toner image is held on the intermediate transfer belt. Yes.

Japanese Patent No. 3772032 JP 2009-204809 A

  However, in the image forming apparatus as described above, when the speed of the intermediate transfer belt is switched during image formation, a speed difference is generated between the photosensitive member and the intermediate transfer belt due to a gear train difference or a motor (drive source) difference. There is. In addition, there is a potential difference between the photosensitive member and the intermediate transfer belt even at the time of switching, and an attractive force acts between the photosensitive member and the intermediate transfer belt due to the potential difference.

  For example, in the image forming apparatus configured as shown in FIG. 7, when the speed of the photosensitive member is reduced first, the intermediate transfer belt 16 is braked by the suction force between the photosensitive member 1 and the intermediate transfer belt 16. As a result, more torque than can be driven is generated, and the drive roller 16a and the intermediate transfer belt 16 slip. Conversely, when the intermediate transfer belt 16 decelerates first, the photoreceptor 1 pulls the intermediate transfer belt 16. At this time, the speed of the intermediate transfer belt 16 becomes faster than the conveying speed of the drive roller 16a, and as a result, the drive roller 16a and the intermediate transfer belt 16 slip. When this slip occurs in a short time, a phenomenon occurs in which the formed toner image and the paper are not aligned. Further, once slip occurs, the frictional force between the drive roller 16a and the intermediate transfer belt 16 decreases, and the drive roller 16a continues to slip as it is, so that the intermediate transfer belt 16 cannot be rotated normally. There was a thing.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus capable of suppressing a slip between a driving roller and an intermediate transfer belt at the time of speed switching and forming a good image.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the first invention is
A photoreceptor,
Charging means comprising a charging member for charging the surface of the photoreceptor;
Exposure means for forming an electrostatic latent image by exposing the surface of the charged photoreceptor;
Developing means for developing the formed electrostatic latent image into a toner image;
In order to transfer the toner image on the photosensitive member to the intermediate transfer belt, a primary transfer unit provided in the intermediate transfer belt and provided with a primary transfer member that presses against the photosensitive member with the intermediate transfer belt interposed therebetween. When,
Have
After the primary transfer of the entire toner image to the intermediate transfer belt, the speed of the intermediate transfer belt is changed according to the type of recording material,
In an image forming apparatus that performs secondary transfer onto the recording material after the toner image has passed the primary transfer position again with the toner image held on the intermediate transfer belt,
Before changing the speed of the intermediate transfer belt, one or both of the voltage applied to the primary transfer member and the surface potential of the photoconductor is controlled, and the potentials of the primary transfer member and the photoconductor surface are controlled. The image forming apparatus is characterized in that the difference is smaller than the potential difference when the toner image on the intermediate transfer belt passes the primary transfer position, and then the speed of the intermediate transfer belt is changed.

The second aspect of the present invention
A photoreceptor,
Charging means comprising a charging member for charging the surface of the photoreceptor;
Exposure means for forming an electrostatic latent image by exposing the charged surface of the photoreceptor;
Developing means for developing the formed electrostatic latent image into a toner image;
A primary transfer member is provided inside the intermediate transfer belt for transferring the toner image of the photoconductor to the intermediate transfer belt, and includes a primary transfer member that presses the photoconductor across the intermediate transfer belt. Means,
Have
In the image forming apparatus in which after the toner image is primarily transferred to the intermediate transfer belt, the speed of the intermediate transfer belt is changed according to the type of the recording material, and the secondary transfer is performed on the recording material.
The image forming apparatus is characterized in that the surface of the photosensitive member is exposed before the speed of the intermediate transfer belt is changed, and the speed of the intermediate transfer belt is changed after the exposed surface reaches the primary transfer position.

  According to the present invention, it is possible to suppress the occurrence of slip between the driving roller and the intermediate transfer belt by reducing the potential difference between the photosensitive member and the intermediate transfer belt when the intermediate transfer belt speed is switched. . Thereby, according to this invention, a favorable image can be formed.

1 is a schematic configuration diagram of an embodiment of an image forming apparatus according to the present invention. It is a schematic block diagram of a primary transfer device. 2 is a block diagram illustrating a configuration of a control unit of the image forming apparatus. FIG. FIG. 4 is a diagram illustrating a speed difference between a photosensitive drum and an intermediate transfer belt when a process speed is switched. It is a flowchart at the time of the process speed switching of one Example. It is a flowchart at the time of the process speed switching of another Example. 1 is a schematic configuration diagram of a conventional image forming apparatus.

  Hereinafter, an image forming apparatus according to the present invention will be described in detail with reference to the drawings.

Example 1
(Overall configuration of image forming apparatus)
FIG. 1 shows a configuration of an image forming apparatus 100 of the present embodiment. In this embodiment, the image forming apparatus 100 is a full-color laser beam printer including a rotary developing device 50.

  The image forming apparatus 100 according to the present exemplary embodiment includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 1 as an image carrier. The photosensitive drum 1 rotates in the direction of the arrow R1. Around the photosensitive drum 1, a charging unit including a charging roller 2 as a charging member and an exposure unit serving as a laser beam scanning device (exposure device) 3 are arranged. Has been. The laser beam L transmitted from the exposure device 3 reaches the exposure position A on the photosensitive drum 1 via the reflection mirror 4 and exposes the photosensitive drum 1.

  The rotary developing device 50 is developing devices 5a, 5b, 5c, and 5d each including yellow toner, magenta toner, cyan toner, and black toner. Since the developing devices 5a, 5b, 5c, and 5d have the same internal configuration, the developing devices 5a, 5b, 5c, and 5d are not distinguished from each other unless the toner contained therein is particularly distinguished. 5 will be described below.

  All the developing devices 5 are configured to be attachable to a rotary 50A as a developing device support. The rotary 50A is rotatably supported in a state where the developing device 5 is mounted, and rotates a desired developing device (for example, the developing device 5a) in the R2 direction to a developing position C that faces and contacts the photosensitive drum 1. Can do.

Below the photosensitive drum 1, an intermediate transfer belt 16 as an intermediate transfer member, which is a transfer material, is stretched around three rollers 16a, 16b, and 16c so as to rotate in the direction R3 in FIG. Has been. One roller 16 a among the rollers that stretch the intermediate transfer belt 16 is a driving roller to which a driving force is transmitted from the driving source M. As the driving roller 16a, a roller member having a diameter of 30 mm and coated with EPDM rubber having a resistance of 10 ⁴ Ω and a thickness of 1.0 mm and having a high frictional force in which carbon is dispersed in an Al cored bar is used. The drive roller 16a rotates the intermediate transfer belt 16 by using the frictional force between the surface of the drive roller 16a and the inner surface of the intermediate transfer belt 16.

  The other rollers are a driven roller 16c and a tension roller 16b, and the tension roller 16b is urged by a tension applying means (not shown) such as a spring so as to apply a predetermined tension to the intermediate transfer belt 16. The tension is 19.6 N on one side and the total pressure is 39.2 N.

  At the primary transfer position B where the photosensitive drum 1 and the intermediate transfer belt 16 are pressed and in contact with each other, a primary transfer device 20 serving as a primary transfer unit is disposed inside the intermediate transfer belt 16. A primary transfer sheet 20 a that is a primary transfer member constituting the primary transfer device 20 is disposed so as to sandwich the intermediate transfer belt 16 with the photosensitive drum 1.

  A secondary transfer roller 18 serving as a secondary transfer unit is disposed so as to face the driving roller 16a and the intermediate transfer belt 16 is sandwiched therebetween, thereby forming a secondary transfer position D. The secondary transfer roller 18 is configured to be able to contact / separate from the intermediate transfer belt 16 at the secondary transfer position D. At the secondary transfer position D, as will be described later, the image is transferred onto the transferred transfer material (recording material) P. The recording material P after the transfer is sent to the fixing device 15.

  A charging roller 19 constituting a secondary transfer residual toner charging device is installed downstream of the secondary transfer position D in the moving direction of the intermediate transfer belt 16. The secondary transfer residual toner charging roller 19 is disposed so as to be able to contact and separate from the intermediate transfer belt 16 in order to charge the secondary transfer residual toner. A photosensitive member cleaning device 9 is installed downstream of the photosensitive drum 1 in the moving direction of the photosensitive drum 1 with respect to the primary transfer position B, and an attached blade contacts the photosensitive drum 1 so as to scrape off the toner. Has been placed.

(Configuration of primary transfer unit)
With reference to FIG. 2, the structure of the primary transfer device 20 which is the primary transfer means in this embodiment will be described.

  The primary transfer device 20 is disposed on the opposite side of the photosensitive drum 1 with the intermediate transfer belt 16 in between, and has a sheet-shaped transfer member (sheet member) 20a as a primary transfer member. The sheet member 20a is pressed against and in contact with the intermediate transfer belt 16 from the opposite side of the photosensitive drum 1 by the pressing member 20d. In this embodiment, ultrahigh molecular polyethylene having a longitudinal width of 230 mm is used as the sheet member 20a.

The volume resistivity of this ultra high molecular weight polyethylene is 10 ³ to 10 ⁴ Ωcm when 5 V is applied, and does not vary greatly from a low temperature and low humidity environment of 15 ° C. and 20% RH to a high temperature and high humidity environment of 30 ° C. and 80% RH. .

  Further, the primary transfer power source 21 shown in FIG. 2 is connected to the sheet member 20a. When a toner image is transferred from the photosensitive drum 1 to the intermediate transfer belt 16, transfer is performed by applying a transfer voltage from the primary transfer power source 21 to the sheet member 20a. One end side of the sheet member 20a is sandwiched and supported by the sheet support member 20b and the seat cover 20c. The seat support member 20b and the seat cover 20c are provided as support means for the seat member 20a.

  In this embodiment, the sheet-shaped sheet member 20a is used as the primary transfer member 20a. However, a roller-shaped roller member or a pad-shaped pad member known to those skilled in the art may be used.

(Intermediate transfer belt)
The intermediate transfer belt 16 can be made of PEN (polyethylene naphthalate) having a thickness of 60 μm and a volume resistivity of 10 ⁶ to 10 ¹¹ Ωcm. In this embodiment, the belt circumferential length was 377 mm.

(Image formation process)
Next, an image forming operation of the image forming apparatus will be described.

  In FIG. 1, the surface of the photosensitive drum 1 rotating at 100 mm / sec in the direction of the arrow R1 is charged to a predetermined potential by a charging roller 2 as a charging member. Specifically, a DC voltage of about −950V to −1200V is applied to the charging roller 2 to charge the surface of the photosensitive drum to about −450 to −600V. At the exposure position A, an electrostatic latent image is formed on the photosensitive drum 1 by the laser beam L transmitted according to the image signal for each color by the exposure device 3 and the reflection mirror 4. The formed electrostatic latent image is developed by the developing device 5 at the development position C to form a toner image. The developing device 5 installed at the developing position C is determined according to the image signal for each color. The developing device 5 of a desired color is installed at the developing position C by rotating the rotary 50A in the arrow R2 direction in advance. To do. The order of colors of toner images to be developed is also determined. In this embodiment, the toner images are formed in the order of yellow, magenta, cyan, and black. Immediately after completion of the developing operation, the rotary 50A is rotated to leave the developing position C.

  The toner image formed on the photosensitive drum 1 is transferred onto the intermediate transfer belt at the primary transfer position B. At this time, a voltage of about 500 to 1200 V is applied to the sheet member 20a which is a primary transfer member. A full color toner image is formed on the intermediate transfer belt by sequentially superimposing the transferred toner image on the transferred toner image. The secondary transfer roller 18 and the secondary transfer residual toner charging roller 19 are separated from the intermediate transfer belt 6 until a full-color toner image is formed, and contacted with the intermediate transfer belt 16 after being formed. The recording material P is conveyed in accordance with the timing at which the formed full color toner image reaches the secondary transfer position D. The secondary transfer roller 18 and the driving roller 16 a sandwich the recording material P together with the intermediate transfer belt 16 to transfer the full color toner image onto the recording material P. The recording material P to which the full color toner image has been transferred is sent to the fixing device 15. The fixing device 15 pressurizes and heats the full-color toner image on the recording material P and fixes it on the recording material P to obtain a final image.

  The toner remaining on the transfer belt 16 in the secondary transfer is charged to a polarity opposite to the normal polarity of the toner by the secondary transfer residual toner charging roller 19 and is electrically reversed on the photosensitive drum 1 at the primary transfer position B. It is copied. Thereafter, the toner is collected by a cleaning device 9 disposed on the photosensitive drum 1.

(Thick paper mode)
In the image forming apparatus 100 of the present embodiment, the process speed after the secondary transfer is variable depending on whether the recording material P to be used is plain paper (normal mode) or thick paper or an OHP sheet (thick paper mode). I have control. Specifically, as shown in FIG. 3, when the type of the recording material P is designated by the user in the image forming apparatus 100, it is determined that the card is in the thick paper mode, and the process speed is, for example, half that in the normal mode. The fixing device 15, the driving roller 16 a, and the driving motor M that applies a rotational force to the photosensitive drum 1 are controlled. Here, the paper type is determined by the user specifying the paper type. However, the present invention is not limited to this, and the image forming apparatus 100 can automatically determine the paper type if a paper type detection unit or the like can be provided. is there.

  As described above, when image formation is performed in the thick paper mode, the process speed (V2) at the time of secondary transfer needs to be half of the process speed (V1) in the normal mode. That is, the process speed must be reduced to V2 after the primary transfer of all the four color toner images onto the intermediate transfer belt 16 and before the secondary transfer onto the recording material P is completed.

  In the configuration of this embodiment, the distance between the primary transfer position B and the secondary transfer position D is shorter than the length of A4 paper in order to achieve downsizing of the apparatus. Therefore, after the completion of the primary transfer for all the toner images, the leading edge of the toner image has already passed the secondary transfer position B at the timing when the process speed is lowered to V2. Therefore, after the process speed is lowered to V2, the intermediate transfer belt 16 is rotated by one revolution, and when the leading edge of the toner image reaches the secondary transfer position D again, secondary transfer is performed on the recording material P. Finally, at least the process speed is maintained at V2 until all the fixing steps are completed.

  Naturally, the switching of the process speed needs to be completed before the leading edge of the toner image reaches the primary transfer position B again when the intermediate transfer belt 16 is idling. This is because if the process speed is switched while the toner image passes through the primary transfer position B, the toner image may be disturbed by a speed difference between the photosensitive drum 1 and the intermediate transfer belt 16 described later.

  When the process speed is switched as described above, a slip occurs between the driving roller 16a and the intermediate transfer belt 16, and the intermediate transfer belt 16 cannot rotate.

  The inventors have found that the above phenomenon has two factors.

  The first factor will be described. When the process speed is switched, the speeds of the photosensitive drum 1 and the intermediate transfer belt 16 are simultaneously switched. However, a speed difference is generated between the photosensitive drum 1 and the intermediate transfer belt 16 due to a gear train difference. As shown in FIG. 4, the timing until the speed is constant and the slope profile are different. In this embodiment, the photosensitive drum 1 is decelerated first, and the intermediate transfer belt 16 is decelerated later. The difference in speed between the photosensitive drum 1 and the intermediate transfer belt 16 at the time of speed switching is the first factor.

  Next, the second factor will be described. It is known that the intermediate transfer belt 16 is electrostatically attracted to the photosensitive drum 1 when there is a potential difference between the surface of the photosensitive drum and the surface of the intermediate transfer belt. This attraction force depends on the potential difference, and the attraction force increases as the potential difference increases. The adsorption force due to this potential difference is the second factor.

  Even if the suction force is high, the drive roller 16a and the intermediate transfer belt 16 do not slip as long as the photosensitive drum 1 and the intermediate transfer belt 16 rotate at substantially the same speed. Conversely, even if there is a speed difference between the photosensitive drum 1 and the intermediate transfer belt 16, no slip occurs if there is no potential difference between the photosensitive drum 1 and the intermediate transfer belt 16. However, when the factors 1 and 2 are combined, slip occurs by the following mechanism.

  In the configuration of the image forming apparatus as shown in FIG. 1, as shown in FIG. 4, when the speed of the photosensitive drum 1 is reduced first, the intermediate transfer belt 16 is braked by the adsorption force between the photosensitive drum 1 and the intermediate transfer belt 16. It takes. As a result, torque exceeding the range in which the driving roller 16a can convey the intermediate transfer belt 16 is generated. Then, the drive roller 16a and the intermediate transfer belt 16 slip.

  It is very difficult to eliminate the speed difference between the photosensitive drum 1 and the intermediate transfer belt 16. In this embodiment, the speed difference is eliminated as much as possible by transmitting the driving force from the same drive motor. However, the speed difference is caused by the difference in the gear train. Although it is conceivable to control the drive source separately, it is difficult to completely eliminate the speed difference in consideration of variations in the drive motor.

(Experimental result)
Therefore, the present inventors conducted an experiment focusing on reducing the potential difference between the photosensitive drum 1 and the intermediate transfer belt 16. The experimental results are shown in Table 1.

  “Charging bias” ON / OFF in the table indicates whether a voltage is applied to charging (charging bias ON) or not. Specifically, the charging bias ON applies a voltage of −950 to −1200 V used during image formation. OFF indicates a state in which the voltage is not applied. “With or without forced light emission” indicates whether or not the photosensitive drum surface is forcibly exposed by the exposure means 3 (with forced light emission). ON / OFF of “primary transfer bias” indicates whether or not a voltage is applied to the primary transfer member (sheet member) 20a (primary transfer bias ON). Specifically, the primary transfer bias ON applies a voltage of 500 to 1200 V that is used for primary transfer during image formation. OFF indicates a state in which the voltage is not applied. “Slip” represents the slip between the driving roller 16a and the intermediate transfer belt 16, and “×” indicates that the intermediate transfer belt 16 is completely slipped and cannot be conveyed and an image cannot be output, or the intermediate transfer belt 16 is short. Shows a state (outside the allowable range on the image) in which the image slips slightly in the transport direction with respect to the paper. “◯” indicates a state where an image problem due to slip does not occur.

  The charging bias and primary transfer bias OFF are not limited to a state where the voltage is completely 0 V, but also include a state where the charging bias and the primary transfer bias are smaller than the absolute value of the voltage applied during image formation.

  With regard to charging bias OFF and forced light emission in the table, the process speed is switched after the photosensitive drum surface on which these are executed reaches the primary transfer position B. The primary transfer bias is turned off before the process speed is switched.

  The condition A in the table is a condition at the time of image formation (until the end of the primary transfer process). If the process speed is switched under these conditions, a slip may occur. However, no slip occurred in B to H in the table, and it was possible to output a very good image. As a result, it has been found that all the operations of charging bias OFF, forced light emission, and primary transfer bias OFF are effective for slip individually.

  First, focusing on the photosensitive drum surface potential (photoconductor surface potential), the operation of each condition will be described. By turning off the charging bias, the potential on the surface of the photosensitive drum (photosensitive member surface) after the completion of the primary transfer is not charged again, so the absolute value of the potential on the surface of the photosensitive drum is lower than that during normal image formation. ing. Further, by performing forced light emission with the charging bias ON, the absolute value of the surface potential of the photosensitive drum is in the following state during image formation. Furthermore, by forcibly emitting light on the surface of the photosensitive drum that has passed with the charging bias OFF, the potential of the photosensitive drum can be lowered to about 0V. Due to these effects, the potential difference between the photosensitive drum and the intermediate transfer belt is smaller than that during image formation.

  In addition, it is clear that the potential difference between the photosensitive drum 1 and the intermediate transfer belt 16 becomes smaller by turning off the primary transfer bias than in the ON state. That is, by controlling one or both of the voltage applied to the primary transfer member 20a and the photosensitive drum surface potential, the difference in potential between the primary transfer member 20a and the photosensitive drum surface can be reduced.

  As described above, B to H in the table have a smaller potential difference between the photosensitive drum 1 and the intermediate transfer belt 16 than A. As a result, the electrostatic attractive force between the photosensitive drum 1 and the intermediate transfer belt 16 is reduced, and slipping is suppressed.

  Examples of the process speed switching sequence will be described as Example A and Example B below.

(Example A)
A process speed switching sequence in the cardboard mode, which is a feature of the present invention in this embodiment, will be described. In the thick paper mode, the following sequence is performed in order to reduce the potential difference between the photosensitive drum 1 and the intermediate transfer belt 16 as much as possible. This will be described with reference to FIG.

  After the development of the final image forming color (Bk) in this embodiment is completed, the black (Bk) developing device is separated from the developing position C by rotary rotation, and then the voltage applied to charging is turned off (S1: charging bias). OFF operation). Next, the potential of the surface of the photosensitive drum (photoconductor surface) is lowered to nearly 0 V by forcibly emitting light (S2: forced light emission operation). Further, after the primary transfer of the entire toner image, the voltage applied to the primary transfer member 20a is turned off (S3: primary transfer bias OFF operation). After turning off the primary transfer bias, the process speed is switched from V1 to V2, and the process speed is changed (S4).

  In the configuration of Example A, after the process speed is switched, the intermediate transfer belt 16 is rotated by one revolution while the toner image is held.

  That is, after changing the process speed, a charging bias is applied (S5), the surface of the photosensitive drum is charged, the forced light emission operation is stopped (S6), and then the primary transfer bias is applied (S7). This state is continued until all the toner images have passed through the primary transfer portion.

  After the process speed is changed, the secondary transfer process and the fixing process are performed in parallel with these sequences. After the image output, the applied voltage such as the charging bias and the primary transfer bias is turned off at a predetermined timing, and the drive motor M is also stopped (S8, S9).

  The order of the charging bias OFF operation of S1 and the forced light emission operation of S2 may be either, but when the surface of the photosensitive drum that has performed either operation reaches the developing position C, the developing device 5 starts from the developing position C. It must be surely withdrawn. By ensuring that the developing device 5 is separated from the developing position C, problems such as fogging do not occur even if the potential of the surface of the photosensitive drum is lowered to about 0 V by performing the above two operations.

  Further, the primary transfer bias OFF operation in S3 is desirably performed after the surface of the photosensitive drum having the lowered surface potential reaches the primary transfer position B. If it is carried out before this timing, the potential of the photosensitive drum 1 cannot be lowered at the primary transfer position B, and the photosensitive drum surface potential remains.

  By turning on the charging bias in S5 and forcibly stopping the light emission in S6, the surface potential of the photosensitive drum is set to the same state as during image formation. The timing of S5 and S6 is such that the photosensitive drum surface charged to the potential during image formation is moved to the primary transfer position before the intermediate transfer belt 16 rotates idly and the toner image reaches the primary transfer position B. It is necessary to set the timing to reach B. Further, the primary transfer bias ON in S7 needs to be performed before the toner image passes through the primary transfer position B again. In these cases, when the intermediate transfer belt 16 rotates idly and the toner image passes through the primary transfer position B again, the toner image is reversely transferred by providing a predetermined potential difference between the photosensitive drum 1 and the intermediate transfer belt 16. It is for suppressing.

  At this time, it is desirable that the voltage applied to the primary transfer member 20a be weaker than that used for primary transfer. This is because there is already a toner image on the intermediate transfer belt 16 and it is sufficient to obtain a potential difference that can be held. If this potential difference is too large, the toner image is reversely charged and the toner is transferred to the photosensitive drum. By reducing this potential difference, reverse charging of the toner image is suppressed, and there is an effect of suppressing transfer of the toner to the photosensitive drum 1.

  By adopting the configuration of the present embodiment A, it is possible to reduce the adsorption force between the photosensitive drum 1 and the intermediate transfer belt 16 by reducing the potential difference between the photosensitive drum 1 and the intermediate transfer belt 16 as much as possible when the process speed is switched. It became. Therefore, even if a speed difference is generated between the photosensitive drum 1 and the intermediate transfer belt 16 when the process speed is switched, the force with which the intermediate transfer belt 16 is pulled by the photosensitive drum 1 is weak. Can be prevented from slipping.

(Example B)
A second embodiment in the process speed switching sequence which is a feature of the present invention will be described with reference to FIG.

  After the development of the final image forming color (Bk) is completed, the black developing device 5 is separated from the developing position C by rotary rotation, and then the exposure is forcibly emitted (S11: forced emission operation), thereby causing the potential of the photosensitive drum surface Is made smaller than that during normal image formation. Thereafter, after the primary transfer of the entire toner image, the voltage applied to the primary transfer member 20a is switched to a bias of about 2/3 times that during normal image formation (S12: primary transfer bias switching operation). . After the primary transfer bias is switched, the process speed is switched from V1 to V2 after the exposure surface of the photosensitive drum 1 reaches the primary transfer position (S13). After switching, the forced light emission operation is stopped (S14).

  In the configuration of Example B, after the process speed is switched, the intermediate transfer belt 16 is rotated by one revolution while the toner image is held. After the process speed is switched, the secondary transfer process and the fixing process are performed in parallel with these sequences. After the image output, the applied voltage such as the charging bias and the primary transfer bias is turned off at a predetermined timing, and the drive motor M is also stopped (S15, S16).

  The step S12 may not be performed depending on the applied voltage.

  By performing the sequence of the embodiment B, the potential difference between the photosensitive drum 1 and the intermediate transfer belt 16 can be reduced when the process speed is switched, and the adsorption force between the photosensitive drum 1 and the intermediate transfer belt 16 can be reduced. It has become possible. As a result, it is possible to suppress the slip between the driving roller 16a and the intermediate transfer belt 16.

  The sequence of the embodiment B is particularly effective in a configuration in which the distance between the primary transfer position B and the secondary transfer position D is close due to miniaturization and the time available for switching the process speed is short. For example, if the time that can be used for switching the process speed is short, such as when the voltage output ON / OFF sensitivity is slow, there is a possibility that all the operations of the embodiment A will not end in time. In such a situation, when the leading edge of the toner image passes through the primary transfer position B, a desired potential difference cannot be provided between the photosensitive drum 1 and the intermediate transfer belt 16, and toner reverse transfer occurs. End up. However, in the case of the sequence of the embodiment B, it is not necessary to reapply the charging bias and the primary transfer bias, and thus the above-described problem does not occur.

1 Photosensitive drum (photoconductor)
2 Charging roller (charging means)
3 Exposure equipment (exposure means)
5 Development device (developing means)
15 Fixing device 16 Intermediate transfer belt (intermediate transfer member)
16a Drive roller 18 Secondary transfer roller (secondary transfer means)
20 Primary transfer device (primary transfer means)
20a Sheet member (primary transfer member)
50 Rotary developing device 50A Rotary

Claims (6)

A photoreceptor,
Charging means comprising a charging member for charging the surface of the photoreceptor;
Exposure means for forming an electrostatic latent image by exposing the surface of the charged photoreceptor;
Developing means for developing the formed electrostatic latent image into a toner image;
In order to transfer the toner image on the photosensitive member to the intermediate transfer belt, a primary transfer unit provided in the intermediate transfer belt and provided with a primary transfer member that presses against the photosensitive member with the intermediate transfer belt interposed therebetween. When,
Have
After the primary transfer of the entire toner image to the intermediate transfer belt, the speed of the intermediate transfer belt is changed according to the type of recording material,
In an image forming apparatus that performs secondary transfer onto the recording material after the toner image has passed the primary transfer position again with the toner image held on the intermediate transfer belt.
Before changing the speed of the intermediate transfer belt, one or both of the voltage applied to the primary transfer member and the surface potential of the photoconductor is controlled, and the potentials of the primary transfer member and the photoconductor surface are controlled. The image forming apparatus is characterized in that the difference is smaller than the potential difference when the toner image on the intermediate transfer belt passes the primary transfer position, and then the speed of the intermediate transfer belt is changed.   The image forming apparatus according to claim 1, wherein the method of controlling the surface potential of the photoconductor reduces an absolute value of a voltage applied to the charging unit.   2. The image forming apparatus according to claim 1, wherein the photoconductor surface potential control method causes the exposure unit to emit light.   The image forming apparatus according to claim 1, wherein the primary transfer member is a sheet member, a roller member, or a pad member. A photoreceptor,
Charging means comprising a charging member for charging the surface of the photoreceptor;
Exposure means for forming an electrostatic latent image by exposing the charged surface of the photoreceptor;
Developing means for developing the formed electrostatic latent image into a toner image;
A primary transfer member is provided inside the intermediate transfer belt for transferring the toner image of the photoconductor to the intermediate transfer belt, and includes a primary transfer member that presses the photoconductor across the intermediate transfer belt. Means,
Have
In the image forming apparatus in which after the toner image is primarily transferred to the intermediate transfer belt, the speed of the intermediate transfer belt is changed according to the type of the recording material, and the secondary transfer is performed on the recording material.
An image forming apparatus, wherein the surface of the photoconductor is exposed before the speed of the intermediate transfer belt is changed, and the speed of the intermediate transfer belt is changed after the exposure surface reaches the primary transfer position.   The image forming apparatus according to claim 5, wherein the primary transfer unit is a sheet member, a roller member, or a pad member.

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